1. Field of the Invention
One disclosed aspect of the embodiments relates to photoelectric conversion apparatuses and image pickup systems and, more particularly, to a photoelectric conversion apparatus including analog-to-digital (AD) converters and an image pickup system including AD converters.
2. Description of the Related Art
Image pickup apparatuses including AD converters are known. Among these image pickup apparatuses, a solid-state image pickup apparatus that uses a plurality of comparators to compare an image signal obtained from each pixel with reference signals that differently change over time and ultimately to obtain a digital signal is described in Japanese Patent Laid-Open No. 2007-281987. In this manner, an output with a high bit precision is obtained in a required range.
In the technique described in Japanese Patent Laid-Open No. 2007-281987, the linearity between an analog signal and a digital signal obtained from the analog signal is not sufficiently discussed. Specifically, in Japanese Patent Laid-Open No. 2007-281987, a difference between slopes (i.e., rates of change with respect to time) of reference signals RAMP1 and RAMP2 respectively supplied to two comparators are set to be equal to an exponential multiple of 2. However, the difference possibly becomes unequal to an exponential multiple of 2 because of, for example, variations in characteristics of a digital-to-analog converter (DAC) that generates the reference signals.